Re: [PATCH for-next 09/14] IB/mlx5: Add support for resize CQ

[Yann Droneaud <ydroneaud-RlY5vtjFyJ3QT0dZR+AlfA@public.gmane.org>]

Hi,

Le mercredi 15 janvier 2014 à 13:24 +0200, Eli Cohen a écrit :
> On Wed, Jan 15, 2014 at 11:02:55AM +0100, Yann Droneaud wrote:
> > 
> > Is libmlx5/libibverbs the ABI ?
> > 
> > Unfortunately, anybody is allowed to access the kernel uverbs API
> > directly, so we must take care of the kernel ABI, just in case.
> >
> 
> I am not sure I understand the question but libibverbs to libmlx5
> relation is somehow like the relationship between ib_core and mlx5_ib.
> e.g. libibverbs sets the API and libmlx5 implements it.

It's a theoretical question:
where to draw the line of the kernel ABI when it comes to uverbs ?
Is it write() + all the data structures for request and response ?
Or is it libibverbs + provider specific library ?

In second case, we can change the behavor of the uverbs provided that it
doesn't interfere with libibverbs: modifying corner case behavors which
were never hit by libibverbs is then safe: it won't break userspace
application.

But in the first case, we cannot for sure attest that modifying behavor
will not break hypothetical userspace application issuing uverbs command
without going through libibverbs: such specific/specialized application
might call uverbs in a way different that libibverbs which could break.

I'm not aware of such unknown application, but I believe it could exists
in the "proprietary" world.

> > 
> > For code not yet part of a released kernel version, we can fix that.
> > But for all other, it would require proper checking/thinking before
> > rejecting reserved field not set to 0 since it might theoterically break
> > existing userspace program: it will be a departure from previous ABI.
> > 
> 
> Yes, this needs to be done carefully for each driver. We need to check
> if either libibverbs or any other provider library clears the struct
> before sending. If this is the case then it should be safe to fix the
> kernel part.
> 
> > 
> > I would found more readable to have the two locks held next each other.
> > YMMV.
> > 
> I am not following you here. Please be more specific.

I thought that reworking the code to move mutex_lock(&cq->resize_mutex)
and spin_lock_irqsave(&cq->lock, flags); next to each other
could show that the first one would not be needed: perhaps is it
possible doing:
 - prepare everything,
 - take the lock,
 - swap,
 - release the lock,
 - clean up.
(oh, that remind me RCU).

I agree that resizing the CQ is not a frequent operation, so trying to
rewrite the code as I suggest won't give a lot of advantages.

Regards.

-- 
Yann Droneaud
OPTEYA


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